Glasses for flat panel displays

ABSTRACT

Glasses are disclosed which are used to produce substrates in flat panel display devices. The glasses exhibit a density less than about 2.45 gm/cm 3  and a liquidus viscosity greater than about 200,000 poises, the glass consisting essentially of the following composition, expressed in terms of mol percent on an oxide basis: 65-75 SiO 2 , 7-13 Al 2 O 3 , 5-15 B 2 O 3 , 0-3 MgO, 5-15 CaO, 0-5 SrO, and essentially free of BaO. The glasses also exhibit a strain point exceeding 650° C.

FIELD OF THE INVENTION

[0001] The present invention relates to alkali-free, aluminosilicateglasses exhibiting desirable physical and chemical properties forsubstrates in flat panel display devices.

BACKGROUND OF THE INVENTION

[0002] Displays may be broadly classified into one of two types:emissive (e.g., CRTs and plasma display panels (PDPs)) or non-emissive.This latter family, to which liquid crystal displays (LCDs) belong,relies upon an external light source, with the display only serving as alight modulator. In the case of liquid crystal displays, this externallight source may be either ambient light (used in reflective displays)or a dedicated light source (such as found in direct view displays).

[0003] Liquid crystal displays rely upon three inherent features of theliquid crystal (LC) material to modulate light. The first is the abilityof the LC to cause the optical rotation of polarized light. Second, isthe ability of the LC to establish this rotation by mechanicalorientation of the liquid crystal. The third feature is the ability ofthe liquid crystal to undergo this mechanical orientation by theapplication of an external electric field.

[0004] In the construction of a simple, twisted nematic (TN) liquidcrystal display, two substrates surround a layer of liquid crystalmaterial. In a display type known as Normally White, the application ofalignment layers on the inner surface of the substrates creates a 90°spiral of the liquid crystal director. This means that the polarizationof linearly polarized light entering one face of the liquid crystal cellwill be rotated 90° by the liquid crystal material. Polarization films,oriented 90° to each other, are placed on the outer surfaces of thesubstrates.

[0005] Light, upon entering the first polarization film, becomeslinearly polarized. Traversing the liquid crystal cell, the polarizationof this light is rotated 90° and is allowed to exit through the secondpolarization film. Application of an electric field across the liquidcrystal layer aligns the liquid crystal directors with the field,interrupting its ability to rotate light. Linearly polarized lightpassing through this cell does not have its polarization rotated andhence is blocked by the second polarization film. Thus, in the simplestsense, the liquid crystal material becomes a light valve, whose abilityto allow or block light transmission is controlled by the application ofan electric field.

[0006] The above description pertains to the operation of a single pixelin a liquid crystal display. High information type displays require theassembly of several million of these pixels, which are referred to assub pixels, into a matrix format. Addressing, or applying an electricfield to, all of these sub pixels while maximizing addressing speed andminimizing cross-talk presents several challenges. One of the preferredways to address sub pixels is by controlling the electric field with athin film transistor located at each sub pixel, which forms the basis ofactive matrix liquid crystal display devices (AMLCDs).

[0007] The manufacturing of these displays is extremely complex, and theproperties of the substrate glass are extremely important. First andforemost, the glass substrates used in the production of AMLCD devicesneed have their physical dimensions tightly controlled. The downdrawsheet or fusion process, described in U.S. Pat. No. 3,338,696 (Dockerty)and U.S. Pat. No. 3,682,609 (Dockerty), is one of the few capable ofdelivering such product without requiring costly post forming finishingoperations, such as lapping and polishing. Unfortunately, the fusionprocess places rather severe restrictions on the glass properties,requiring relatively high liquidus viscosities, preferably greater than200,000 poises.

[0008] Typically, the two substrates that comprise the display aremanufactured separately. One, the color filter plate, has a series ofred, blue, green, and black organic dyes deposited on it. Each of theseprimary colors must correspond precisely with the pixel electrode areaof the companion, active, plate. To remove the influence of differencesbetween the ambient thermal conditions encountered during themanufacture of the two plates, it is desirable to use glass substrateswhose dimensions are independent of thermal condition (i.e., glasseswith lower coefficients of thermal expansion). However, this propertyneeds to be balanced by the generation of stresses between depositedfilms and the substrates that arise due to expansion mismatch. It isestimated that an optimal coefficient of thermal expansion is in therange of 28-33×10⁻⁷/° C.

[0009] The active plate, so called because it contains the active, thinfilm transistors, is manufactured using typical semiconductor typeprocesses. These include sputtering, CVD, photolithography, and etching.It is highly desirable that the glass be unchanged during theseprocesses. Thus, the glass needs to demonstrate both thermal andchemical stability.

[0010] Thermal stability (also known as thermal compaction or shrinkage)is dependent upon both the inherent viscous nature of a particular glasscomposition (as indicated by its strain point) and the thermal historyof the glass sheet as determined by the manufacturing process. U.S. Pat.No. 5,374,595, disclosed that glass with a strain point in excess of650° C. and with the thermal history of the fusion process will haveacceptable thermal stability for active plates based both on a-Si thinfilm transistors (TFTs) and super low temperature p-Si TFTs. Highertemperature processing (such as required by low temperature p-Si TFTs)may require the addition of an annealing step to the glass substrate toensure thermal stability.

[0011] Chemical stability implies a resistance to attack of the variousetchant solutions used in the manufacture processes. Of particularinterest is a resistance to attack from the dry etching conditions usedto etch the silicon layer. To benchmark the dry etch conditions, asubstrate sample is exposed to an etchant solution known as 110BHF. Thistest consists of immersing a sample of glass in a solution of 1 volumeof 50 wt. % HF and 10 volumes 40 wt. % NH₄F at 30° C. for 5 minutes. Thesample is graded on weight loss and appearance.

[0012] In addition to these requirements, AMLCD manufacturers arefinding that both demand for larger display sizes and the economics ofscale are driving them to process larger sized pieces of glass. Currentindustry standards are Gen III (550 mm×650 mm) and Gen III.5 (600 mm×720mm), but future efforts are geared toward Gen IV (1 m×1 m) sizes, andpotentially larger sizes. This raises several concerns. First andforemost is simply the weight of the glass. The 50+% increase in glassweight in going from Gen III.5 to Gen IV has significant implicationsfor the robotic handlers used to ferry the glass into and throughprocess stations. In addition, elastic sag, which is dependent uponglass density and Young's Modulus, becomes more of an issue with largersheet sizes impacting the ability to load, retrieve, and space the glassin the cassettes used to transport the glass between process stations.

[0013] Accordingly, it would be desirable to provide a glass compositionfor display devices having a low density to alleviate difficultiesassociated with larger sheet size, preferably less than 2.45 g/cm³ and aliquidus viscosity greater than about 200,000 poises. In addition, itwould be desirable for the glass to have thermal expansion between about28-35×10⁻⁷/° C., and preferably between about 28-33×10⁻⁷/° C., over thetemperature range of 0-300° C. Furthermore, it would be advantageous forthe glass to have a strain point greater than 650° C., and for the glassto be resistant to attack from etchant solutions.

SUMMARY OF THE INVENTION

[0014] The present invention is founded in the discovery of glassesexhibiting densities less than 2.45 g/cm³ and a liquidus viscosity(defined as the viscosity of the glass at the liquidus temperature)greater than about 200,000 poises, preferably greater than about 400,000poises, more preferably greater than about 600,000 poises, and mostpreferably greater than about 800,000 poises. Additionally, the glassesof the present invention exhibit linear coefficients of thermalexpansion over the temperature range of 0-300° C. between about28-35×10⁻⁷/° C., and preferably between about 28-33×10⁻⁷/° C., andstrain points higher than about 650° C. The glass of the presentinvention has a melting temperature less than about 1700° C. Inaddition, the glass exhibits a weight loss of less than about 0.5 mg/cm²after immersion in a solution of 1 part HF 50 wt. % and 10 parts 40% wt.% NH₄F for 5 minutes at 30° C.

[0015] The glass of the present invention has a composition consistingessentially of the following composition as calculated in mole percenton an oxide basis: 65-75 SiO₂, 7-13 Al₂O₃, 5-15 B₂O₃, 0-3 MgO, 5-15 CaO,0-5 SrO, and essentially free of BaO. More preferably, the glass of thepresent invention has a composition consisting essentially of thefollowing composition as calculated in mole percent on an oxide basis:67-73 SiO₂, 8-11.5 Al₂O₃, 8-12 B₂O₃, 0-1 MgO, 5.5-11 CaO, and 0-5 SrO.

[0016] We have discovered that for glasses having the compositions andphysical properties discussed above, especially the preferredcompositions and preferred properties, the liquidus viscosity of theglass is strongly influenced by the ratio of the sum of alkaline earths,RO (R═Mg, Ca, Sr) to alumina on a mol % basis, orRO/Al₂O₃═(MgO+CaO+SrO)/Al₂O₃. This ratio is referred to as RO/Al₂O₃, andshould be held in the range 0.9 to 1.2. Most preferably, this rangeshould be 0.92<RO/Al₂O₃<0.96 to obtain the highest liquidus viscosity.

[0017] The glasses of the present invention are essentially free of BaO,which means that the glasses preferably contain less than about 0.1 mol% BaO. The glasses of the invention are also essentially free of alkalimetal oxides, which means that the glasses preferably contain a total ofless than about 0.1 mol % of alkali metal oxides. Additionally, theseglasses may contain fining agents (such as the oxides of arsenic,antimony, cerium, tin, and/or the halides, chlorine/fluorine).

[0018] In another aspect of the invention, the glasses have a meltingtemperature less than about 1700° C. The glasses of the presentinvention also exhibit a weight loss of less than 0.5 mg/cm² afterimmersion in a solution of 1 part 50 wt. % HF and 10 parts 40% wt. %NH₄F for 5 minutes at 30° C. The glasses are useful as a substrate forflat panel displays. Substrates made from the glass of the presentinvention have an average surface roughness as measured by atomic forcemicroscopy of less than about 0.5 nm and an average internal stress asmeasured by optical retardation of less than about 150 psi.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention is concerned with improved glasses for useas flat panel display substrates. In particular, the glasses meet thevarious property requirements of such substrates.

[0020] The preferred glasses in accordance with the present inventionexhibit a density of less than about 2.45 gm/cm³, preferably less thanabout 2.40 gm/cm³, a CTE over the temperature range of 0-300° C. betweenabout 28-35×10⁻⁷/° C., preferably between about 28-33×10⁻⁷/° C., andstrain points higher than about 650° C., preferably greater than 660° C.A high strain point is desirable to help prevent panel distortion due tocompaction/shrinkage during subsequent thermal processing.

[0021] For more demanding manufacturing conditions such as the fusionprocess, described in U.S. Pat. No. 3,338,696 (Dockerty) and U.S. Pat.No. 3,682,609 (Dockerty), a glass having a high liquidus viscosity isrequired. Therefore, in a preferred embodiment of the present invention,the glasses should exhibit a density less than about 2.45 gm/cm³ and aliquidus viscosity greater than about 200,000 poises, preferably greaterthan about 400,000 poises, more preferably greater than about 600,000poises, and most preferably greater than about 800,000 poises. Althoughsubstrates made from glass of the present invention can be made usingother manufacturing processes such as the float process, the fusionprocess is preferred for several reasons. First, glass substrates madefrom the fusion process do not require polishing. Current glasssubstrate polishing is capable of producing glass substrates having anaverage surface roughness greater than about 0.5 nm (Ra), as measured byatomic force microscopy. Glass substrates produced according to thepresent invention and using the fusion process have an average surfaceroughness as measured by atomic force microscopy of less than 0.5 nm.

[0022] Chemical durability involves a resistance to attack of thevarious etchant solutions used in the manufacture processes. Ofparticular interest is a resistance to attack from the dry etchingconditions used to etch the silicon layer. One benchmark of the dry etchconditions is exposure to a etchant solution known as 110BHF. This testconsists of immersing a sample of glass in a solution of 1 volume of 50wt. % HF and 10 volumes 40 wt. % NH₄F at 30° C. for 5 minutes. Chemicalresistance is determined by measuring weight loss in terms of mg/cm².This property is listed in Table I as “110 BHF.”

[0023] The glasses of the present invention include 65-75 mol %,preferably 67-73 mol %, SiO₂ as the primary glass former. Increasingsilica improves the liquidus viscosity and reduces the density and CTEof the glass, but excessive silica is detrimental to the meltingtemperatures. The glasses also comprise 7-13 mol %, preferably 8-11.5mol %, Al₂O₃. Increases in Al₂O₃ content increase glass durability anddecrease CTE, but liquidus temperatures increase. At least 8 mol % isrequired to have the most desired strain point; however, more than 11.5mol % results in a less than desired liquidus temperature.

[0024] The glasses further include 5-15 mol %, preferably 8-12 mol %boric oxide. Boric oxide lowers the liquidus temperature and density andpreferably is present in at least 8 mol %; however, more than 12 mol %boric oxide will negatively impact the glass strain point.

[0025] MgO is present in the glasses of the present invention in anamount of 0-3 mol %, preferably 0-1 mol %. Increasing MgO decreasesliquidus viscosity, and therefore, no more than 3 mol % MgO should bepresent in the glass. However, smaller amounts of MgO may be beneficialfor reducing density.

[0026] CaO is useful to lower both the melting and liquidus temperaturesof the glass; however, more than 11 mol % will result in a less thandesired strain point and a higher than desired coefficient of thermalexpansion. Therefore, the glasses of the present invention can include5-15 mol % CaO, but preferably include 5.5-11 mol % CaO.

[0027] The RO/Al₂O₃ ratio is in the range 0.9-1.2. Within this rangelocal minima in liquidus temperature can be found, corresponding tocotectics and/or eutectics in the base system CaO—Al₂O₃—SiO₂. Theviscosity curves of the glasses in this series do not varysignificantly, thus these changes in liquidus temperatures are theprimary drivers for increases in liquidus viscosity.

[0028] Because of their negative impact on thin film transistor (TFT)performance, alkalis such as lithia, soda or potash are excluded fromthe present glass compositions. Likewise, heavier alkaline earth metals(SrO and BaO) will be either minimized or excluded because of theirnegative impact on the glass density. Accordingly, the glasses of thepresent invention may include 0-5 mol % SrO. However, the glasses of theinvention are essentially free of BaO. As used herein, essentially freeof BaO means that the composition contains less than 0.1 mol % of BaO inthe composition.

[0029] Fining agents such as As₂O₃, Sb₂O₃, CeO₂, SnO₂, Cl, F, SO₂, etc.may also be present to aid in the removal of seeds from the glass. Theglass may also contain contaminants as typically found in commerciallyprepared glasses. In addition, the following oxides can be added at alevel not exceeding 1 mol % without pushing properties outside of theranges described above: TiO₂, ZnO, ZrO₂, Y₂O₃, La₂O₃. Table I listsexamples of glasses of the present invention in terms of mol % alongwith their physical properties. The comparative example in Table 1 isCorning Incorporated's 1737 glass.

[0030] The invention is further illustrated by the following examples,which are meant to be illustrative, and not in any way limiting, to theclaimed invention. TABLE I sets forth exemplary glass compositions inmol percent, as calculated on an oxide basis from the glass batches.These example glasses were prepared by melting 1,000-25,000 gram batchesof each glass composition at a temperature and time to result in arelatively homogeneous glass composition, e.g. at a temperature of about1625° C. for a period of about 4-16 hours in platinum crucibles. Alsoset forth are relevant glass properties for each glass composition,determined on the glasses in accordance with techniques conventional inthe glass art. Thus, the linear coefficient of thermal expansion (CTE)over the temperature range 0-300° C. is expressed in terms of×10⁻⁷/° C.,the softening point (Soft. Pt.), and the annealing point (Ann. Pt.), andstrain point (Str. Pt.) are expressed in terms of ° C. These weredetermined from fiber elongation techniques (ASTM references E228-85,C338, and C336, respectively). The density (Den.), in terms of g/cm³,was measured via the Archimedes method (ASTM C693).

[0031] The 200 poise temperature (Melt. Temp., ° C.) (defined as thetemperature at which the glass melt demonstrates a viscosity of 200poises [20 Pa.s]) was calculated employing the Fulcher equation fit tothe high temperature viscosity data (measure via rotating cylindersviscometry, ASTM C965-81). The liquidus temperature (Liq. Temp.) of theglass was measured using the standard liquidus method. This involvesplacing crushed glass particles in a platinum boat, placing the boat ina furnace having a region of gradient temperatures, heating the boat inan appropriate temperature region for 24 hours, and determining by meansof microscopic examination the highest temperature at which crystalsappear in the interior of the glass. The liquidus viscosity (Liq. Visc.,in poises) was determined from this temperature and the coefficients ofthe Fulcher equation.

[0032] Table I records a number of glass compositions, expressed interms of parts by mole on the oxide basis, illustrating thecompositional parameters of the present invention. Inasmuch as the sumof the individual constituents totals or very closely approximates 100,for all practical purposes the reported values may be deemed torepresent mole percent. The actual batch ingredients may comprise anymaterials, either oxides, or other compounds, which, when meltedtogether with the other batch components, will be converted into thedesired oxide in the proper proportions. For example, SrCO₃ and CaCO₃can provide the source of SrO and CaO, respectively.

[0033] Glasses having the compositions and properties shown in Examples14 and 19 are currently regarded as representing the best mode of theinvention, that is, as providing the best combination of properties forthe purposes of the invention at this time.

[0034] Although the invention has been described in detail for thepurpose of illustration, it is understood that such detail is solely forthat purpose and variations can be made therein by those skilled in theart without departing from the spirit and scope of the invention whichis defined by the following claims. TABLE 1 Composition Batched (mol %)1 2 3 4 5 6 7 8 9 SiO₂ 69.75 69.78 69.75 69.8 70.7 70.57 69.1 70.85 69.1Al₂O₃ 10.2 10.2 10.3 10.3 10 9.77 10.35 9.5 10.35 B₂O₃ 9.7 9.7 9.5 9.510 9.95 10.25 10.1 10.25 MgO 0.8 0.8 0.8 0.8 0.12 0.15 1.25 0.15 CaO 7 78.5 8.5 9 9.17 9.8 6.7 9.3 SrO 2.2 2.2 0.8 0.8 1.3 0.5 BaO As₂O₃ 0.3 0.30.4 0.33 0.33 Sb₂O₃ 0.3 0.3 0.3 0.1 CeO₂ 0.2 Y₂O₃ SnO₂ 0.05 0.02 0.050.05 0.02 0.02 0.02 Cl 0.2 0 0.2 0.2 0.2 RO/Al₂O₃ 0.98 0.98 0.98 0.980.90 0.95 0.96 0.98 0.96 CTE (0-300° C., × 32.7 31.2 31.9 30.8 30.5 30.231.5 31.4 32.2 10⁻⁷/° C.) Dens. (g/cm³) 2.416 2.404 2.394 2.38 2.372.355 2.375 2.367 2.377 Str. Pt (° C.) 672 674 673 679 671 666 673 664671 Ann. Pt. (° C.) 727 731 729 734 728 729 729 720 727 Soft. Pt. (° C.)984 991 991 995 1001 1001 976 992 983 110 BHF (mg/cm²) 0.15 0.14 0.1450.135 0.14 0.15 0.18 0.27 0.15 Liq. Temp. (° C.) 1125 1120 1120 11251150 1135 1115 1140 1095 Liq. Visc. (p) 5.04E + 05 7.06E + 05 6.34E + 056.30E + 05 3.66E + 05 5.95E + 05 5.30E + 05 4.77E + 05 1.05E + 06 Melt.Temp. (° C.) 1659 1668 1650 1659 1668 1680 1635 1686 1649 CompositionBatched (mol %) 10 11 12 13 14 15 16 17 18 SiO₂ 69.65 68.75 69.1 68.869.33 69.65 70.05 69.33 69.45 Al₂O₃ 10.1 10.55 10.2 10.35 10.55 10.2 9.910.55 11 B₂O₃ 10.25 10.25 10.55 10.55 9.97 10.25 10.25 9.97 9 MgO 0.150.15 0.15 0.15 0.18 0.15 0.15 0.18 CaO 9.45 9.9 9.65 9.8 9.08 8.85 9.259.58 10 SrO 0.5 0.5 BaO As₂O₃ 0.4 0.4 0.33 0.33 0.37 0.4 0.4 0.37 Sb₂O₃0.3 CeO₂ Y₂O₃ SnO₂ 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.05 Cl 0.2RO/Al₂O₃ 0.95 0.95 0.96 0.96 0.93 0.93 0.95 0.93 0.91 CTE (0-300° C., ×31.4 31.8 32 31.8 31.5 30.9 30.8 31.4 32 10⁻⁷/° C.) Dens. (g/cm³) 2.3662.371 2.364 2.366 2.376 2.369 2.357 2.366 2.386 Str. Pt (° C.) 669 674666 667 676 671 668 674 681 Ann. Pt. (° C.) 725 729 722 723 731 728 726730 737 Soft. Pt. (° C.) 985 986 980 980 985 987 992 987 993 110 BHF(mg/cm²) 0.12 0.12 Liq. Temp. (° C.) 1090 1100 1090 1115 1095 1100 11001120 1150 Liq. Visc. (p) 1.23E + 06 9.19E + 05 1.07E + 06 5.74E + 051.19E + 06 1.04E + 06 1.12E + 06 6.23E + 05 2.90E + 05 Melt Temp. (° C.)1654 1650 1642 1650 1651 1657 1674 1654 1642 Composition Batched (mol %)19 20 21 22 23 24 25 26 27 SiO₂ 69.33 68.3 70.45 70 70.4 70.1 70 68.3 68Al₂O₃ 10.55 10.9 9.85 10.35 10.05 10 10.23 10.9 10.5 B₂O₃ 9.97 10.4 9.559.5 9.69 10.15 9.75 10.4 11 MgO 0.18 1.1 0.8 0.82 0.81 0.8 CaO 9.08 9.15.75 8.5 7.14 7.11 7 9.1 10.5 SrO 0.5 1 2.9 0.85 1.89 1.88 2.22 1.0 BaOAs₂O₃ 0.4 0.3 Sb₂O₃ 0.37 0.3 0.3 CeO₂ 0.2 0.2 0.2 0.2 Y₂O₃ SnO₂ 0.020.05 0.05 Cl 0.2 RO/Al₂O₃ 0.93 0.93 0.99 0.98 0.98 0.98 0.98 0.93 1.00CTE (0-300° C., × 31.9 34.8 33.4 32.8 33.5 32.9 33.4 34.5 34.9 10⁻⁷/°C.) Dens. (g/cm³) 2.383 2.403 2.405 2.378 2.378 2.374 2.391 2.392 2.378Str. Pt (° C.) 675 669 670 672 670 664 671 677 659 Ann. Pt. (° C.) 730725 727 728 727 720 727 732 713 Soft. Pt. (° C.) 984 978 996 988 989 984989 979 967 110 BHF (mg/cm²) 0.16 0.3 0.39 0.18 0.17 0.19 0.19 0.23 Liq.Temp. (° C.) 1100 1140 1165 1160 1140 1150 1150 1150 1120 Liq. Visc. (p)1.06E + 06 2.40E + 05 2.88E + 05 2.49E + 05 4.29E + 05 2.80E + 053.11E + 05 2.14E + 05 3.07E + 05 Melt. Temp. (° C.) 1655 1617 1682 16561668 1653 1656 1620 1611 Composition Batched (mol %) 28 29 30 31 32 CompEx SiO₂ 70 70.4 69.8 71 72 67.6 Al₂O₃ 10 10.2 10.6 9.9 9.33 11.4 B₂O₃ 109.1 9 10 10 8.5 MgO 1.3 CaO 9 5.68 7.14 9.1 8.66 5.2 SrO 4.55 3.42 1.3BaO 4.3 As₂O₃ 0.4 Sb₂O₃ 0.3 0.3 0.3 0.3 0.3 CeO₂ Y₂O₃ SnO₂ Cl RO/Al₂O₃0.90 1.00 1.00 0.92 0.93 1.03 CTE (0-300° C., × 32.0 34.4 34.4 32.1 31.237.8 10⁻⁷/° C.) Dens. (g/cm³) 2.355 2.420 2.406 2.353 2.342 2.54 Str. Pt(° C.) 673 669 672 669 672 666 Ann. Pt. (° C.) 731 727 729 726 730 721Soft. Pt. (° C.) 999 987 984 998 1012 975 110 BHF (mg/cm²) 0.18 0.2 Liq.Temp. (° C.) 1130 1130 1135 1120 1125 1050 Liq. Visc. (p) 5.99E + 055.67E + 05 4.38E + 05 8.46E + 05 8.79E + 05 2.97E + 06 Melt. Temp. (°C.) 1670 1671 1657 1685 1703 1636

What is claimed is:
 1. An aluminosilicate glass exhibiting a densityless than about 2.45 g/cm³ and a liquidus viscosity greater than about200,000 poises, the glass consisting essentially of the followingcomposition as calculated in mol percent on an oxide basis: 65-75 SiO₂,7-13 Al₂O₃, 5-15 B₂O₃, 0-3 MgO, 5-15 CaO, 0-5 SrO, and essentially freeof BaO.
 2. The glass of claim 1, wherein the RO/Al₂O₃ ratio is between0.9 and 1.2, wherein R represents Mg, Ca, Sr and Ba.
 3. The glass ofclaim 1, wherein the glass has a strain point greater than about 650° C.4. The glass of claim 1, wherein the glass has a linear coefficient ofthermal expansion (CTE) over the temperature range 0-300° C. between28-35×10⁻⁷/° C.
 5. The glass of claim 4, wherein the glass has a strainpoint greater than about 660° C.
 6. The glass of claim 4, wherein theglass has a melting temperature less than about 1700° C.
 7. The glass ofclaim 4, wherein the glass has a CTE of 28-33×10⁻⁷/° C.
 8. The glass ofclaim 1, wherein the glass exhibits a weight loss of less than 0.5mg/cm² after immersion in a solution of 1 part 50 wt. % HF and 10 parts40 wt. % NH₄F for 5 minutes at 30° C.
 9. The glass of claim 1, whereinthe glass has a liquidus viscosity greater than about 400,000 poises.10. A glass according to claim 1, wherein the glass has a liquidusviscosity greater than about 600,000 poises.
 11. A glass according toclaim 1, wherein the glass contains between 0-1 mole percent MgO whenthe glass contains no SrO.
 12. In a flat panel display device, theimprovement comprising a substrate in accordance with claim
 1. 13. Theflat panel display device of claim 12, wherein the substrate has anaverage surface roughness less than about 0.5 nm.
 14. The flat paneldisplay device of claim 12, wherein the substrate has an averageinternal stress less than about 150 psi.
 15. A glass according to claim1, wherein the glass has a composition consisting essentially of, asexpressed in mol percent on an oxide basis: 67-73 SiO₂, 8-11.5 Al₂O₃,8-12 B₂O₃, 0-1 MgO, 5.5-11 CaO, and 0-5 SrO.
 16. The glass of claim 15,wherein the glass has a strain point greater than about 650° C.
 17. Theglass of claim 15, wherein the glass has a CTE of 28-33×10 ⁻⁷/° C. 18.The glass of claim 17, wherein the glass has a strain point greater thanabout 660° C.
 19. The glass of claim 17, wherein the glass has a meltingtemperature less than about 1700° C.
 20. The glass of claim 17, whereinthe glass has a liquidus viscosity greater than 400,000 poises.
 21. Theglass of claim 17, wherein the glass has a liquidus viscosity greaterthan about 800,000 poises
 22. In a flat panel display device, theimprovement comprising a substrate in accordance with claim
 17. 23. Theflat panel display device of claim 22, wherein the substrate has anaverage surface roughness less than about 0.5 nm.
 24. The flat paneldisplay device of claim 22, wherein the substrate has an averageinternal stress less than about 150 psi.
 25. In a flat panel displaydevice, the improvement comprising a substrate in accordance with claim21.
 26. A substrate for a flat panel display device, wherein thesubstrate is comprised of a flat, transparent glass exhibiting a densityless than about 2.40 g/cm³, a linear coefficient of thermal expansion(CTE) over the temperature range 0-300° C. between 28-33×10⁻⁷/° C. andhaving a liquidus viscosity greater than about 400,000 poises, the glassconsisting essentially of the following composition as calculated in molpercent on an oxide basis: 65-75 SiO₂, 7-13 Al₂O₃, 5-15 B₂O₃, 0-3 MgO,5-15 CaO, 0-5 SrO, and essentially free of BaO and the RO/Al₂O₃ ratio is0.92-0.96, wherein R represents Mg, Ca, Sr, and Ba.
 27. A substrateaccording to claim 26, wherein the glass exhibits a strain pointexceeding 660° C.
 28. The substrate according to claim 26, wherein thesubstrate has an average surface roughness less than about 0.5 nm. 29.The substrate according to claim 26, wherein the substrate has anaverage internal stress less than about 150 psi.